This invention relates in general to current collection systems for high power dynamoelectric machines, and more particularly, to brush to shunt attachments for current collection systems. The invention is particularly applicable to homopolar generators and particularly to those of the advanced kind having very high power and continuous duty over the lifetime of the generator.
Brush to shunt attachments have heretofore comprised a composite brush, such as copper with a small percent by weight of graphite and other volatile components. This is conductively interfaced through either a resiliently loaded conductive interface, or alternatively, with the individual brushes each in a bonded interface with a non-composite metallic shunt such as a copper conductor which conducts the current to a load circuit.
The spring loaded brush to shunt is used in a brush box approach represented by FIG. 4, in U.S. Pat. No. 4,710,665, entitled "Homopolar Dynamoelectric Machine With Self-Compensating Current Collector" which is assigned to the same assignee as the present invention. One of the problems with this type of brush to shunt interface is that the spring contact fingers of the shunt have a relatively high contact resistance, and also that this problem is aggravated by the less than permanent spring constant and other aging effects which are not preventable. The above-mentioned patented approach describes a pulse duty type current generation system. Another type is a propulsion type or continuous duty current generation system.
Heretofore, the typical joined brush to shunt attachments were made by tamping, riveting, or soldering since the graphite and other volatile components of a composite brush do not lend themselves to effective welding or brazing. One of the principal problems with welding is that the volatile components of the brush, during these attachments, are driven off by the elevated temperatures at which the bonding process for the brush to shunt interface occurs. Another problem is that this driving off effect precludes the wetting of the brush and shunt materials. The volatilized components degrade the conductivity and contribute to a high resistivity in the interface of the brush to shunt attachment. The high resistivity in the interface results in a high temperature rise in the brush during operation which severely limits the performance of the brush. Therefore, the brush becomes inadequate to meet the demands of continuous duty, high powered homopolar power supplies.
Another problem is that the brush to shunt interface is normally weakened by the formation of voids or cracks which are initiated by the interfacial melting temperature layers presented during the process of brazing or soldering. The shrinkage stresses combined with the initiation of voids or cracks, promotes delamination of the joint during service conditions which demand a low and stable temperature rise in the brush so as not to limit the performance requirements for a high continuous duty.
Likewise tamping or riveting of the brush to shunt joint does not provide adequate strength to withstand cyclic contact and withdrawal from the current collection zone which represents a normal mode of positional change. This degrades the mechanical joint of the brush to shunt attachment resulting in a weakened interface and a corresponding deterioration in current collecting performance. A performance criterion which the prior art brush to shunt attachments do not fully satisfy is to avoid the limitations on generator performance which is imposed by the brush to shunt attachment zone rather than the more basic limitations imposed by the current collection zone in the primary interface with the spinning rotor.